1. Field of the Invention
The present invention relates to a soft magnetic low-carbon steel useful for forming iron cores for solenoids, relays and solenoid valves to be applied to various electric devices for automobiles, electric trains and ships, a method of manufacturing the soft magnetic low-carbon steel, and a method of manufacturing a soft magnetic low-carbon part of the soft magnetic low-carbon steel. More particularly, the present invention relates to a soft magnetic low-carbon steel excellent in cold forgeability, machinability and magnetic characteristic, and a method of manufacturing a soft magnetic low-carbon steel part of the soft magnetic low-carbon steel having an excellent magnetic characteristic.
2. Description of the Related Art
Component members of magnetic circuits included in electric devices for automobiles and such are required to have a low coercive force, in addition to a capability of being easily magnetized by a low-intensity external magnetic field, for the improvement of power consumption and response characteristic of the electric circuits. Thus, those component members of magnetic circuits are formed of soft magnetic materials so that the magnetic flux density in those component members changes in quick response to the change of an external magnetic field. Representative soft magnetic steels are very-low-carbon steels having a carbon content on the order of 0.01% by mass (hereinafter, content is expressed in percent by mass, unless otherwise specified). A soft magnetic steel part is manufactured by subjecting a steel billet of a very-low-carbon steel to hot rolling to obtain a steel sheet, and sequentially subjecting the steel sheet to lubrication, drawing, cold forging (or cold pressing), finish machining and magnetic annealing.
There is a tendency for the shape and construction of soft magnetic steel parts to become complicated to cope with the development of high-performance electric devices in the recent years. While on the one hand the very-low-carbon steel is excellent in cold press workability, the very-low-carbon steel is very liable to form flashes and burrs when a workpiece of the very-low-carbon steel is subjected to a shearing process or a drilling process. Consequently, a very-low-carbon steel part having a complicated shape is difficult to machine and cannot manufacture at high productivity.
Under such circumstances, some measures have been pro-posed to improve the machinability of soft magnetic steels. An invention disclosed in, for example, JP51-16363B relating to a method of improving the machinability of pure-steel soft magnetic material adds a low-melting metal, such as Pb or Bi in a proper content to the pure-steel soft magnetic material to improve the machinability of the pure-steel soft magnetic material and to extend the life of tools without deteriorating the magnetic characteristic of the pure-steel soft magnetic material. The principal object of this previously proposed invention, however, is to improve the life of tools, and the previously proposed invention is not necessarily satisfactory in effect of reducing the formation of burrs during machining. The element added to the soft magnetic material to improve the machinability of the soft magnetic material affects adversely to the magnetic characteristic of the soft magnetic material. Thus, the magnetic characteristic of the soft magnetic material containing such an additive element is JIS SUYB Class 2, at the most.